In recent years the need for rapid, high volume liquid/liquid separation for the oil industry has increased. Most of the secondary and tertiary recovery methods developed in the United States and Canada utilize large quantities of water, resulting in mixtures of oil and water which are difficult to separate. Even the major oil producing countries in the Middle East are now beginning to produce ever increasing volumes of brine with their oil, increasing the demand for high volume desalting equipment.
New developments and processes in other industries are also requiring liquid/liquid separation equipment. A good example in the mining industry is the solvent extraction processes. The chemical industry is utilizing similar liquid ion exchange processes. All of these processes require elaborate liquid/liquid mixing and/or separating facilities.
The use of high voltage electric fields to force the separation of oil field emulsions is a well known and accepted practice. These fields greatly speed the coalescence and separation of immiscilbe liquids, over conventional heater treaters and settlers using mechanical aids to coalescense. However, considerable retention time is still necessary, and large vessels are required if large volumes of emulsions are to be processed in a short time.
"Retention Time" is that period required for a first fluid dispersed in a second fluid to settle into a single body from which it can be removed. Many things will affect retention time. A large factor is the size of the drops formed by the dispersed fluid. Considering gravity to be the usual external force applied to the dispersed drops, if the diameter of these drops are doubled, their falling velocity through the fluid in which the drops are dispersed will be increased ten times under Stokes Law. An electric field is a tool which has been used to increase the size of the dispersed drops by forcing separated drops to join each other, or coalesce. The increase in the falling velocity of the coalesced drops will enable the size of the retaining vessel required for retention time to be greatly reduced.
One of the problems in using the electric field is centered in its strength. When the field begins to coalesce the dispersed drops its force upon there enlarging drops greatly increases. With the field strength constant, the enlarged drops travel in the second fluid fast enough to develop shear forces with the liquid in which they are dispersed to separate, or fragment, the enlarged drops. The conclusion is that the drops then broken up by the very force that caused them to coalesce. The electric field should be adjusted in strength as the fluid coalesces to larger size drops to prevent this cycle of coalescense ending in re-dispersion. It is the constant strength presently maintained for electric fields which limits the size of the drops of dispersed fluid.
Some values should be used to analyze the problem more completely. Drop size has been indicated as a large factor in liquid-liquid separation. Proximity of the drop to an electrode of the field is also considered. The larger the drop size, and the closer the drop to the electrode, the less electrical force is required to move the drop relative the electrode. Density and viscosity of the mixed fluids will affect mobility of the drops dispersed. However, as an example, if water drops in the order of 2-5 mirons in size were moved by an electric field strong enough to coalesce the drops, that same force would set up a limiting liquid shear force which would keep the size of the drops to the order of 6-10 microns. Water drops of this size will not settle from a light oil and separate in a practical period of retention time.
The strength of the electric field for the water-light oil mixture must be in the order of 30-40 thousand volts per inch. From this value, the field will have to be reduced after coalescense begins in order to prevent the shear forces from breaking up and redispersing them.